Color temperature is a physical quantity used in lighting optics for defining the color of the light source. The color temperature is defined as follows: heating a black-body to a certain temperature, when the color of the emitted light is the same as the color of the light emitted by a light source, the temperature of the black-body heating is called the color temperature of the light source, color temperature for short. The unit is expressed in “K” (Kelvin temperature unit). For general people, a low color temperature light source is usually called warm color, generally appeared as red, yellow or orange. A high color temperature light source is usually called cold color, generally appeared as blue or purple. The color temperature of some common light source, for example, standard candle is 1930K (Kelvin temperature unit); tungsten wire is 2760-2900K; fluorescent lamp is 6400K; flash is 3800K; noon sun is 5000K; electronic flash is 6000K; blue sky is 10000K.
Modern lighting equipment has evolved into being made of LED. Many are composed of LED string. The present white LEDs mostly are made by coating a layer of pale yellow phosphor on a blue LED (near-UV, wavelength is from 450 nm to 470 nm). LED string emit first, and then illuminate to the phosphor, so that it looks white. However, if the product design specifications require a certain stable color temperature, or a particular color temperature curve, for LED string designers, it will be an important challenge. The entire LED industry needs an LED with adjustable color temperature, to allow designers to design and manufacture high-quality adjustable color temperature LED string easily. In addition, for the natural light, the color temperature often changes as the brightness. How to reduce cost and simulate natural light have always been a technical challenge.